Horford sat down for the last time against Chicago with 4:07 left in the third quarter and his team leading by 37 points. He was a plus-31 in the game and became one of 16 active players to reach 10,000 points while averaging no more than 11.4 field-goal attempts.

He’s also the 14th in NBA history among players who were named to at least five All-Star teams. Horford and the Charlotte Hornets’ Dwight Howard are the only two active players on that list.

Since early in the season, Al Horford has been critical to Boston's success.

That economy of impact -- scoring when needed, as part of an overall game -- illustrates the value Horford has brought since his rookie season with the Atlanta Hawks in 2007-08. But it never has been more apparent than in the 146 games, regular and postseason, he has played for the Celtics since signing a four-year, $113 million free-agent deal with them in July 2016.

That seemed awfully pricey for a guy who had averaged more than 18 points or 10 rebounds once each in his first nine seasons. But Boston improved from 48 victories and a first-round exit in 2015-16 to 53 and the Eastern Conference finals last season. This time, the Celtics are on pace to win 57, with playoff ambitions to match or top 2017’s.

At 12.8 points per game, Horford (who has scored 20 or more 10 times this season) only ranks fourth among Celtics scorers, averaging barely half of Kyrie Irving’s 24.8 ppg. But it was that way a year ago, too, with Isaiah Thomas at 28.9 ppg and Horford at 14.0 ppg.

In fact, one can make the case that Horford prefers it so, allowing him to contribute in a multitude of ways while still commanding respect from opposing defenses -- and obviously, All-Star reserve votes from the conference’s coaches. His net rating is 7.1, compared to Irving’s 4.9.

I came to Boston for the opportunity to compete for a championship. As long as we’re winning, that’s what I care about. It’s always been that way with me, but it’s even more true now."

Celtics center Al Horford

“If Al is open, we’re going to get him the ball,” Celtics coach Brad Stevens said. “If Al is at the rim, we want to get him the ball. But Al is also OK being a facilitator and making plays for everybody else.

“Al is a lot more concerned with how our team plays than his stat line.”

See some of the plays that made Al Horford an All-Star in 2018.

Even when Horford strung together some meager stats lines after the All-Star break -- three points at Detroit, five points each against Memphis and Charlotte -- Boston still won. It is 5-1 since the break, 2 1/2 games behind the Toronto Raptors for top spot in the East … and 6 1/2 in front of third-place Cleveland.

Those are the sort of numbers that get Horford’s attention.

“I came to Boston for the opportunity to compete for a championship,” he said on his way out of United Center. “As long as we’re winning, that’s what I care about. It’s always been that way with me, but it’s even more true now.

“Kyrie’s the leader of our team, the head of our team. And for me, it’s just to make sure I’m making things easy for him. Then when the team needs me to step up and score, I will. Otherwise I need to just defend.”

Don’t be fooled -- Horford can exploit defensive mismatches and lapses, and rise to the proper occasion (he had 21 points, nine rebounds and 10 assists in Game 1 of the 2017 Eastern Conference semifinals and 15-6-5 in the Game 7 clincher).

Now that he’s on my team, you can see all the things he brings. His work ethic, and why he’s been in the league so long.”

Celtics guard Marcus Smart, on Al Horford

But there are a dozen other things Horford does in the seams, on and especially off the court, to help steer the Celtics. Not the least of which is his presence as a grown-up in the room, so often underrated in this league.

Horford was old beyond his years, a calming voice in Atlanta’s 74 playoff games from 2008-16. He’s that guy again, only more so.

“This season for me has been up and down,” Smart said. “Injuries and things like that. He’s just always been there. Always positive. You need that when you go through hard times. He’s been coaching up our rookies, helping them along the way. It’s tough being a rookie, especially on a good team. You get thrown into the fire, you need somebody like Al there to help you.”

Teammate Marcus Morris referred to Horford as “the ultimate vet.” But like Smart, he didn’t know much previously about Horford’s personality or character. To Morris, Horford seemed a fairly non-descript opponent.

It wasn’t until Horford signed with Boston that Smart got to know him, with Morris getting a better glimpse since his trade from Detroit last summer.

“What’s funny is, he’s kind of a quiet guy,” Smart said. “So I wasn’t really expecting much. [As an opponent] you couldn’t really tell. But now that he’s on my team, you can see all the things he brings. His work ethic, and why he’s been in the league so long.”